Rock Mechanics.pdf - Mining and Blasting

Figure 4.49 Composite peak
strength characteristics for a hypothetical
rock mass containing four
sets of discontinuities each with the
properties of the cleavage in the slate
for which the data shown in Figure
4.33b were obtained.
ROCK STRENGTH AND DEFORMABILITY
be assumed to be isotropic. This would not be the case if one of the discontinuity sets
had a substantially lower shear strength than the other sets.
Because of the difficulty of determining the overall strength of a rock mass by
measurement, empirical approaches are generally used. As discussed in section 4.5.5,
Brady (1977) found that the power law of equation 4.23 could be applied to the
mineralised shale at the Mount Isa Mine. An attempt to allow for the influence of
rock quality on rock mass strength was made by Bieniawski (1976) who assigned
Coulomb shear strength parameters, c and , to the various rock mass classes in
his geomechanics classification (Table 3.5). Correlations have also been proposed
between other rock mass classification schemes and rock mass strengths (e.g. Barton,
2002, Laubscher, 1990, Laubscher and Jakubec, 2001).
The most completely developed of these empirical approaches is that introduced
by Hoek and Brown (1980). Because of a lack of suitable alternatives, the Hoek-
Brown empirical rock mass strength criterion was soon adopted by rock mechanics
practitioners, and sometimes used for purposes for which it was not originally intended
and which lay outside the limits of the data and methods used in its derivation. Because
of this, and as experience was acquired in its practical application, a series of changes
were made and new elements introduced into the criterion. Hoek and Brown (1997)
consolidated the changes made to that time and gave a number of worked examples to
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